This Jan. 8, 2015 photo shows a realty sign in front of a home for sale in Jackson, Miss. Freddie Mac reports on average U.S. mortgage rates on Thursday, March 19, 2015. (AP Photo/Rogelio V. Solis)

WASHINGTON — Average long-term U.S. mortgage rates fell this week, remaining near historically low levels at the start of the spring home-buying season.

Mortgage giant Freddie Mac said Thursday the national average for a 30-year fixed-rate mortgage declined to 3.78 percent from 3.86 percent last week.

The average rate for a 15-year mortgage, popular with homeowners who refinance, slipped to 3.06 percent from 3.10 percent last week.

A year ago, the average 30-year mortgage stood at 4.32 percent and the 15-year mortgage at 3.32 percent. Mortgage rates have remained low even though the Federal Reserve in October ended its monthly bond purchases, designed to hold down long-term rates.

The Fed has kept short-term rates near zero for over six years, and signaled Wednesday that it's still not ready to start raising them. The statement the Fed issued after its latest policy meeting indicated that it needs the job market to improve further and inflation to rise above low levels before it begins nudging rates up. Even then, the Fed suggested it will do so only very gradually.

To calculate average mortgage rates, Freddie Mac surveys lenders across the country at the beginning of each week. The average doesn't include extra fees, known as points, which most borrowers must pay to get the lowest rates. One point equals 1 percent of the loan amount.

The average fee for a 30-year mortgage was 0.6 point, unchanged from last week. The fee for a 15-year mortgage also remained at 0.6 point.

The average rate on a five-year adjustable-rate mortgage fell to 2.97 percent from 3.01 percent. The fee was stable at 0.5 point.

For a one-year ARM, the average rate was unchanged at 2.46 percent. The fee remained at 0.4 point.